The present invention relates to a transmitter according to the preamble of claim 1, a method for signal processing in a transmitter according to the preamble of claim 22, a computer program according to claim 39 and a computer readable medium according to claim 41.
“Transmitter” is here to be given a very broad sense and also covers so called transmitter configurations having for example a plurality of branches for said communication signal and/or means for transmitting the transmission signal, such as antennas, as for example an adaptive antenna. However, it is pointed out that the transmission medium does not necessarily have to be “the air”, but it could be any conceivable medium, such as a cable. Furthermore, “communication signal” is defined as the signal including the information to be transmitted by the transmitter travelling to and past the power amplifier, and the “transmission signal” is defined as the signal at the output of said power amplifier transmitted by the transmitter and also extracted for creating the feedback signal and travelling in a feedback loop back to said means for signal parameter adaptation.
A transmitter of this type may find may preferred applications, among which as a transmitter for mobile base stations may be mentioned as a non-limitative example.
The general construction of such a transmitter according to the prior art is illustrated in the appended FIG. 1 and will now be briefly explained with reference made thereto. This transmitter has a signal source 1, which can be digital or analog, a frequency up-conversion device 2, a power amplifier 3 and an antenna 28 adapted to transmit the transmission signal resulting at the output of the power amplifier 3. The frequency up-conversion device has preferably at least two mixing stages each including a local oscillator 4, 5 and a frequency mixer 6, 7. Filters 8, 9 are arranged in the signal path downstream of each frequency mixer for rejecting unwanted spurii and only letting for instance the wanted sideband through. The respective filter requirements may be reduced if the frequency mixer in question is instead a single sideband mixer or quadrature modulator configuration having then two frequency mixers. The reason for using multiple stages in the frequency up-conversion device is to spread the filtering requirements between different mixing stages, such that the overall filtering requirements can be met. If instead only one mixing stage was used, the filter thereof would have to reject all unwanted spurii, some of them very close to the wanted signal. Such a filter is very hard to design, especially at high frequencies. With more than one mixing stage, the filtering requirements will be distributed and therefore be easier met, so that filters with high selectivity work at lower frequencies making the design thereof much easier.
For some transmitters high demands on certain properties of the transmission signal are put, and they therefore require a feedback observation signal from the output of the power amplifier for enabling adjustment of the communication signal for obtaining these properties of the transmission signal. The transmission signal is for this sake extracted at the output of the power amplifier 3 by an extracting means 10, which may be for example a coupler, but it may also be for example an antenna receiving the transmission signal transmitted by the antenna 28. A frequency down-conversion of this extracted transmission signal then takes place in a frequency down-conversion device 11 using the same local oscillators 4, 5 as the frequency up-conversion device 2 making the design coherent. A design having separate local oscillators for the frequency up- and down-conversion would also be coherent if the local oscillators were locked to the same time reference. This also means a lower cost, since the number of local oscillators is reduced. A feedback signal having the same frequency as the input signal to the frequency up-conversion device is in this way obtained, and this signal is then compared with a communication signal corresponding to a wanted transmission signal for signal parameter adaptation of the communication signal. The comparison has to be made in this low frequency region, but the adjustments of the communication signal may be carried out anywhere along the path thereof, such as in this low frequency region, or even between the frequency up-conversion device and the power amplifier or after the latter. A block 84 between the coupler 10 and antenna 28 is also shown. This block may have nothing in it, or it may have a duplexer if a receiver is to be attached to the same antenna, or it may have a circulator if it is desired to protect the integrity of the feedback down-conversion signal from external signals picked up by the antenna or a combination of the two.
Said comparison may be made continuously, but it is underlined that the signal parameter adaptation is only carried out as often as required when the condition changes for creating a transmission signal having the desired properties for these new conditions. In one possible application a predistortion of the communication signal, in the digital or in the analogous domain, is carried out for cancelling distortions of the signal generated in the communication signal path, such as in the power amplifier, for obtaining a substantially distortion-free transmission signal. These distortions may change with for example temperature and component ageing, so that an adaptation of the predistortion parameters therefore is needed. Another possible application is in a transmitter comprising an adaptive antenna, in which said comparison is utilized to influence the communication signal for meeting gain and/or phase requirements of the transmission signals from different antennas thereof. Such a transmitter comprises a plurality of branches each including a frequency up-conversion device, power amplifier and an antenna, but the invention also covers the case of such a multiple branch configuration in which all the branches have one up-converter in common, or one up-converter and one power amplifier in common.
A problem with a transmitter of the type illustrated in FIG. 1 is that the transmission signal down-conversion path is unnecessarily complex and contains a lot of components in the signal path. This leads to linear as well as non-linear distortion of the signal, distorting the feedback-signal so created at the output of the frequency down-conversion device. This makes it troublesome to achieve an accurate copy of the signal at the output of the power amplifier for said comparison reducing the accuracy thereof and thereby the success of the performance-enhancing adjustments of the transmission signal.
U.S. Pat. No. 4,700,151 describes a modulation system capable of improving a transmission system, in which a feedback signal is created through frequency down-conversion of an output signal from a power amplifier for predistortion of a communication signal in the communication signal path for compensating for non-linearities of the amplified output signal. This transmitter uses the same local oscillator for the up- and down-conversion. However, this transmitter only uses one mixing stage.
Furthermore, U.S. Pat. No. 5,974,302 discloses a transceiver, in which the receiver and the transmitter uses the same local oscillators, and the receiver as well as the transmitter thereof have more than one frequency conversion stage. U.S. Pat. No. 5,937,011 describes a transceiver including dual stage up- and down-conversion and common local oscillators as defined in the preamble of appended claim 1.